Paraneoplastic pemphigus (PNP) is recognised in most cases after diagnosis of malignant and benign haematological tumours. PNP usually presents with severe and diffuse oral ulcerations, ocular lesions, lichen planus-like skin lesions and frequently genital ulcerations. We describe the uncommon case of a patient unaware of any neoplasia with a unique ulcerated oral lesion with histological (acantholysis of the basal epithelial layer, necrotic keratinocytes and pronounced regenerative hyperplasia) and immunofluorescent (direct immunofluorescence test exhibited immunoglobulin IgG, fibrinogen and C3 deposition in intercellular areas and along the basement membrane; indirect immunofluorescence test performed on rat bladder showed bright fluorescence) features suggestive of PNP. Diagnosis of PNP was strengthened by the subsequent discovery of monoclonal gammopathy. The reported case is quite unusual if we consider the clinical appearance of the oral lesions and the patient's negative medical history. Following serological examinations, the patient proved to have monoclonal gammopathy of undetermined significance (MGUS), one of the most common premalignant plasma cell disorders.